Electron microscopy is a well-known technique for imaging microscopic objects. The basic genus of electron microscope has undergone evolution into a number of well-known apparatus species, such as the Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), and Scanning Transmission Electron Microscope (STEM), and also into various sub-species, such as so-called “dual-beam” tools (that additionally employ a “machining” beam of ions, allowing supportive activities such as ion-beam milling or ion-beam-induced deposition, for example). In traditional electron microscopes, the imaging beam is “on” for an extended period of time during a given imaging session; however, electron microscopes are also available in which imaging occurs on the basis of a relatively short “flash” or “burst” of electrons, such an approach being of potential benefit when attempting to image moving samples or radiation-sensitive specimens, for example.
In current electron microscopes [and other charged-particle microscopes], use is often made of a detector that employs an evacuated photo-multiplier tube (PMT) in conjunction with a scintillator. In such a set-up, output electrons emanating from the sample move toward and strike the scintillator (which will often be maintained at an accelerating potential of the order of a few kV with respect to the sample), thus causing the production of photonic radiation (i.e. electromagnetic radiation, such as visible light) that, in turn, is directed (e.g. with the aid of a light guide) to a photo-emissive cathode of the PMT, from which it triggers the ejection of one or more photoelectrons. Each such photoelectron traverses a series of high-voltage dynodes—each of which emits a plurality of electrons for each impinging electron (cascade effect)—so that a greatly augmented number of electrons eventually leaves the last dynode and strikes a detection anode, producing a measurable electric current or pulse. The cathode, dynodes and anode are all located in an evacuated vitreous tube.
This known detector set-up (often referred to as an Everhart-Thornley detector) has certain drawbacks. For example, the vitreous tube of the PMT is necessarily quite bulky, seeing as it has to accommodate multiple electrodes in a specific mutual configuration, and has to support high internal vacuum. Such bulkiness is exacerbated by the fact that each electrode requires an electrical connection through the wall of the vitreous tube to the tube's exterior, where it is connected via an electrical cable to a high-voltage source (typically operating in the kV range). In addition, a light guide between the scintillator and the PMT may necessarily be quite long (e.g. due to spatial restrictions in placement of the PMT), and this will generally lead to some degree of signal loss. Moreover, the very principle of operation of the PMT results in a relatively large ultimate electrical current for each electron that strikes the scintillator; consequently, in a scenario in which irradiation of a sample by an imaging beam produces a relatively large flux of output radiation from the sample, this can result in an excessive electrical current at the anode of the PMT. To mitigate this effect, one can attempt to attenuate the input to the PMT in some way, e.g. by making the employed scintillator less sensitive, but such action will generally tend to complicate the detector set-up even further.
Accordingly, there is a need to provide a radically alternative detection scenario to that set forth above.